A GENETIC STUDY OF NORTH AMERICAN MELANTHACEJE. X341 
A Systematic Study of the North American Melantbacere 
from the Genetic Standpoint. By R. Ruaaius Gares, Ph.D., F.L.S. 
(With Map (Plate 5) and 1 text-figure.) 
[ Read 15th March, 1917.] 
INTRODUCTION. 
THE purpose of the present paper is a little different from that of any 
previous publication, so far as the writer is aware. It aims to furnish the 
applieation to a particular group of plants of a definite point of view 
regarding the genesis of species. It may be said that every systematist has 
a point of. view in his work, and this is doubtless true, the point of view 
having been furnished largely by the general Darwinian background, which 
assumes the gradual and infinitesimal differentiation of species. 
The point of view of the present work is essentially different, in that it 
assumes the variations which differentiate species not to have been universally 
continuous and infinitesimal, but to have been definite and often discontinuous. 
To the writer, one of the most important advances since Darwin has been in 
the discovery of the definiteness * of variation. This knowledge is based 
upon experimental results obtained largely through the work of De Vries, 
Bateson, Baur, Nilsson-Ehle, and many others during the last fifteen years. 
The writer has endeavoured to analyze this definiteness in the case of the 
(Enothera mutations, and has reached the conclusion + that many different 
though definite types of change are involved, each giving rise to a different 
and characteristic mutation. The essential conceptions of variation thus 
developed through long experience and experiment by the writer with the 
(Enotheras are here applied to the specific and generic differentiation of a 
group of Liliales which has not been made the subject of experiment. In 
short, the method is to apply the mutation conceptions to the systematic 
treatment of this Liliaceous group in order to determine in how far the 
method is valid and useful. It cannot be expected that final conclusions will 
be reached in the absence of experimental analysis, anl yet I believe the 
method is sufficiently illuminating to justify its adoption and application to 
other groups. On the other hand, it must obviously be used with caution, 
and in such a way that speculation does not too far outrun verification by 
means of cytological studies and breeding experiments. If, however, the 
* By definiteness of variation I mean not necessarily orthogenetie variation, but marked 
and clearly defined variations which may occur in any direction or in many directions 
simultaneously. : 
+ ‘The Mutation Factor in Evolution, p. 853. Macmillans, London, 1915. 
LINN, JOURN.—BOTANY, VOL. XLIV. M 
